The present invention relates to methods of driving plasma display panels and, more particularly, to methods of driving AC discharge plasma display panels having electrodes covered by a dielectric and operable in indirect AC discharge state.
The plasma display panel (hereinafter referred to as PDP) usually has many advantages, such as having a thin structure, being free from flicker, having a large contrast ratio, being capable of relatively readily providing a large display area, being able to provide fast response, and being of self light emission type to permit multiple color light emission by utilizing phosphors. Owing to these advantages, the PDP is recently finding wide-spread applications to the fields of displays concerning computers and color image displays.
PDPs are classified depending on their operating system into an AC discharge type having electrodes covered by a dielectric and operable in an indirect AC discharge state, and a DC discharge type having electrodes exposed to a discharge space and operable in a DC discharge state. The AC discharge PDPs have advantages in eliminating spattering of electrodes due to discharge and long life. The AC PDPs are further classified depending on the drive system into a memory type which utilizes a discharge cell memory, and a refresh type which does not utilize any such memory.
The PDP light intensity is proportional to the number of discharge times, i.e., the number of repeated pulse voltage applications. In the refresh type PDP, the light intensity is reduced with increasing display capacity. Therefore, this type is adopted for only small discharge capacity PDPs.
FIG. 7 is a sectional view showing an example of the AC discharge memory type PDP.
As shown in the FIG. 7, this PDP comprises a rear and a front insulating substrate 1 and 2 of glass, a scan and a sustaining electrode 3 and 4, which are transparent and formed at a predetermined spacing therebetween on the substrate 2 and constitute each of a plurality of parallel electrode sets, tracing electrodes 5 and 6 formed on the scan and sustaining electrodes 3 and 4 for reducing the electrode resistance thereof, a dielectric layer 12 covering the scan, sustaining and tracing electrodes 3 to 6, a protective layer 13 of magnesium oxide or like material laminated on the dielectric layer 12 for protecting the same layer 12 from discharge, a plurality of parallel data electrodes 7 (only one thereof being shown) formed on the substrate 1 such as to cross the scan and sustaining electrodes 3 and 4, a dielectric layer 14 covering the data electrodes 7, a plurality of discharge gas spaces 8 (only one thereof being shown) formed between the substrates 1 and 2 and filled with discharge gas, e.g., of helium, neon, xenon, etc. or a mixture of these gases, a partitioning wall member 9 provided on the dielectric layer 14 to form the discharge gas spaces 8 and define display cells, and phosphor 11 coated on the dielectric layer 14 and also on the side wall surfaces of the partitioning wall member 9 for converting ultraviolet rays generated with discharge of the discharge gas filled in the discharge gas spaces 8 to visible light 11.
The actual PDP, for instance a VGA panel, has a cell pixel matrix on the display area, having 480 rows or lines and 1,920 columns of display cells or pixels like those noted above, 480 scan electrodes 3 and the same number of sustaining electrodes 4, and 1,920 data electrodes 7. As the pitch of pixels, the data electrode pitch is 0.35 mm, and the scan electrode pitch is 1.05 mm. The scan electrodes are at a distance of 0.2 mm from the data electrodes, and the scan and sustaining electrodes 3 and 4 in each electrode set are spaced apart by 0.1 mm.
The discharge operation of the PDP having the above construction will now be described.
By applying a pulse voltage in excess of a discharge threshold level is applied between the scan and data electrodes 3 and 7, discharge is initiated. As a result, positive and negative charges are attracted to and deposited on either ones of the surfaces of the opposite side dielectrics 12 and 14 in dependence on the polarity of the pulse voltage. The equivalent internal voltage that arises from the charge deposition, i.e., wall voltage, has the opposite polarity to the applied pulse voltage. For this reason, with the progress of discharge, the effective voltage across the cell is reduced. Therefore, in spite of the constant pulse voltage level, it eventually becomes unable to sustain discharge, resulting in de-sustaining thereof.
By subsequently applying a sustaining pulse, which is a pulse voltage of the same polarity as the wall voltage, between the scan and sustaining electrodes 3 and 4, the wall voltage is superimposed as effective voltage on the sustaining pulse. Thus even if the voltage amplitude of the externally applied sustaining pulse is low, the voltage across the cell is held above the discharge threshold level, and the discharge can be sustained.
The discharge thus can be sustained by continuously applying the sustaining pulse between the scan and sustaining electrodes 3 and 4.
The sustained discharge can be de-sustained by applying, to the scan or sustaining electrode 3 or 4, an erasing pulse, which is a wide low voltage pulse or a narrow pulse with a level of the order of the sustaining pulse voltage such as to neutralize or cancel the wall voltage.
FIG. 8 is a schematic plan view showing the PDP having the display cell matrix array as shown in FIG. 7.
As shown in FIG. 8, the PDP 15 is a dot matrix display panel having display cells 16 in a dot matrix array of j rows and k columns. As row electrodes, the PDP has parallel scan electrodes S.sub.c1, S.sub.c2, . . . , S.sub.cj and sustaining electrodes S.sub.u1, S.sub.u2, . . . , S.sub.uj and as column electrodes it has data electrodes D.sub.1, D.sub.2, . . . , D.sub.k crossing the scan and sustaining electrodes.
FIG. 9 is a waveform chart showing a prior art example of drive pulses for driving the PDP shown in FIG. 8. This example is proposed in Society for Information Display International Symposium Digest of Technical Papers, Vol. XXVI, 1995, pp. 807-810.
In the FIG. 9, labeled W.sub.u is a sustaining electrode drive voltage waveform applied commonly to the sustaining electrodes S.sub.u1, S.sub.u2, . . . S.sub.uj. Labeled W.sub.s1, W.sub.s2, . . . W.sub.sj are scan electrode drive voltage waveforms applied to the scan electrodes S.sub.c1, S.sub.c2, . . . , S.sub.cj, respectively. Labeled W.sub.d is a data electrode drive voltage waveform applied to the data electrode D.sub.i (123 i.ltoreq.k).
As shown in FIG. 9, the PDP drive cycle consists of a priming discharge period A, a writing discharge period B and a sustaining period C. Desired image display can be obtained with repeated discharge in these periods.
In the priming discharge period A, active particles and wall charge are generated in each discharge gas space 8 (see FIG. 7) in order to obtain stable writing discharge characteristics in the writing discharge period B. In this period, a priming discharge pulse P.sub.p is first applied to the sustaining electrodes to simultaneously cause discharge in all the display cells of the PDP 15. Then, a priming discharge erase pulse P.sub.pe is simultaneously applied to all the scan electrodes to remove or reduce those charge parts of the wall charge generated in this period which impede the writing discharge and the sustained discharge.
Specifically, the priming discharge pulse P.sub.p is first applied to the sustaining electrodes S.sub.u1, S.sub.u2, . . . , S.sub.uj cause discharge in all the cells, and then the priming discharge erase pulse P.sub.pe is applied to the scan electrodes S.sub.c1, S.sub.c2, . . . , S.sub.cj to bring about erase discharge for erasing the charge parts of the charge deposited by the priming discharge pulse P.sub.p which impede the writing discharge and the sustained discharge.
In the writing discharge period B, a scan base pulse P.sub.b is first applied to all the scan electrodes S.sub.c1, S.sub.c2, . . . , S.sub.cj. Then, successive scan pulses P.sub.w are applied to the scan electrodes S.sub.c1, S.sub.c2, . . . , S.sub.cj, and in synchronism to the scan pulses P.sub.w data pulses Pd are selectively applied to the data electrodes D.sub.i (1.ltoreq.i.ltoreq.k) in the display cells to be driven for display. Thus, writing discharge is caused to generate wall charge in the cells to be driven.
By applying the scan base pulse P.sub.b, it is possible to reduce the level of the scan base pulse P.sub.w, thus reducing the maximum working voltage level of a high break-down voltage IC which generates the scan pulses P.sub.w and reducing the cost of the drive IC, when the level of the scan pulses P.sub.w is high, the rising of the scan pulse P.sub.w causes undesired discharge tending to de-sustain the writing discharge, which is brought about by the scan and data pulses. By applying the scan base pulse P.sub.b, the level of the scan pulse P.sub.w can be reduced to eliminate the undesired discharge.
In the sustaining period C, a sustaining pulse P.sub.u is applied to the sustaining electrodes, while applying a sustaining pulse P.sub.s lagging in phase by 180 degrees behind the sustaining pulse P.sub.u to each scan electrode, thus sustaining discharge necessary to obtain a desired light intensity in the display cells, in which the writing discharge was brought about in the writing discharge period B.
A method of gradation display with the above PDP will now be described.
In the PDP, unlike other devices, it is difficult to obtain high light intensity gradation display by causing applied voltage changes. Generally, therefore, the gradation display is obtained by controlling the number of times by which to cause light emission. Particularly, a sub-field method as will be described in the following is used for high light intensity gradation display.
FIG. 10 is a view for describing the sub-field method. In the Figure, the ordinate is taken for the scan electrode column, and the abscissa is taken for time.
Usually, one frame image is set during one filed as shown in FIG. 10. One field is largely set to be in a range of 1/47 to 1/76 second, although it varies with computers and broadcasting systems.
In the gradation image display using the PDP, as shown in FIG. 10, one field is divided into k sub-fields (i.e., sub-fields SF1 to SF6, k=6 in the case shown in FIG. 10), each sub-field being constituted by one PDP drive cycle as shown in FIG. 9.
The light intensity of emission in each pixel in each sub-field is controlled by weight multiplying the number of times of sustained discharge light emission in the pixel by 2.sup.n, and is given as: ##EQU1##
In formula (1), n is sub-field number, being "1" for the lowest light intensity sub-field and "k" for the highest light intensity sub-field. L.sub.1 is the light intensity of the lowest light intensity sub-field. a.sub.n is a variable having value of either "1" or "0" and being "1" when driving pertinent pixels in the n-th sub-field for light emission and "0" otherwise. Since the light intensity of emission varies with the sub-fields, the light intensity can be controlled by selecting "on" and "off" states in each field.
The sub-fields shown in FIG. 10 have different time lengths. This is so because the number of times of the sustained discharge, i.e., the number of applied sustaining pulses P.sub.u and P.sub.n, is different with different sub-fields.
In the case shown in FIG. 10, k=6. When this case is applied to color display with a red, a green and a blue color pixel as a set, a gradation display of 2.sup.k =2.sup.6 =64 gradations is obtainable. Also, it is possible to obtain a display in 64.sup.3 =262,144 different colors (including black). In case of k=1, one field is constituted by one sub-field, and two gradations (i.e., "on" and "off" gradations) are obtainable in each color. Also, a display in 2.sup.3 =8 different colors (including black) is obtainable.
In the PDP, its electrodes are formed on insulating substrate of glass and covered by glass graze, which is a dielectric constituted by glass paste. For this reason, a phenomenon called electromigration takes place, which is the precipitation of a glass graze component on the electrode surface due to an effective DC bias applied between the scan and sustaining electrodes. By the term "effective DC bias" is meant not a purely DC bias but a voltage deviation in the integration of the voltage applied between the scan and sustaining electrodes over one drive cycle.
FIG. 11 is a view showing an example of voltage waveforms, which brings about the electromigration.
When the voltages of the waveforms as shown in FIG. 11 are applied to the scan and sustaining electrodes, respectively, it reduces to the equivalence, when viewed from the scan or sustaining electrode side, that a negative bias of -50 V is applied for 10 msec. to the scan electrode.
In the meantime, for obtaining a gradation display of 62 gradations, at least 6 drive cycles are necessary in a field as shown in FIG. 10. When the image is to be re-written at 60 Hz, it is equivalent to 6.times.60 DC bias application periods in one second.
This DC bias brings about electromigration, i.e., migration of ions in the direction of the electric field. As a result, projections like tree branches grow between the scan and sustaining electrodes, and ultimately cause short-circuit thereof with one another.
Even without short-circuit caused between the scan and sustaining electrodes, the projections grown thereon are of a conductive material, and increase the effective electrode area. This means that the effective discharge gap between the scan and sustaining electrodes is reduced.
Such effective discharge gap reduction extremely reduces the initial discharge voltage in a short period of time. That is, even when a voltage which permits normal image display is initially set, erroneous "on" pixels are caused in a short period display operation. This erroneous "on" pixels disable maintaining normal display operation.
A measure for preventing the above phenomenon is described in Japanese Patent Laid-Open Publication No. 8-160909. FIG. 12 is a view showing an example of drive waveforms for preventing the electromigration.
In the Figure, designated at A is a priming discharge period, at B a writing discharge period, at C a sustaining period, at P.sub.p a priming discharge pulse, at P.sub.b a scan base pulse, at P.sub.w scan pulses, at P.sub.x a pulse of voltage of 50 V, at P.sub.u sustaining electrode side sustaining pulses, and P.sub.s scan electrode side sustaining pulses.
In this example, of the scan electrode side sustaining pulses applied in the sustaining period C, the last one is continued into a pause period. This last pulse cancels a DC voltage that is applied between the scan and sustaining electrodes in the writing discharge period B.
In this prior art PDP drive method, in which a sustaining pulse applied in the pause period cancels the DC voltage applied between the scan and sustaining electrodes in the writing discharge period, the pause period should be set to be equal in time length to the writing discharge period. However, the pause period may not always be set to be equal to the writing discharge period In other words, the pause period may be set to be equal to the writing discharge period with a sacrifice in the sustaining period. In such a case, the result may be the failure of obtaining sufficient light intensity.
This prior art method has another problem. The voltage of the last sustaining pulse applied to the scan electrode side, is different from the voltage applied between the scan and sustaining electrodes in the writing discharge period. However, the effect of electromigration varies non-linearly with respect to the last applied sustaining pulse voltage. Therefore, it is difficult to determine the level and pulse length of the compensation pulse voltage, which is applied to the scan electrode side during the pause period in order to perfectly cancel the DC bias voltage in the writing discharge period.